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seismic oceanography. In one of the seminal seismic ocean- ography papers, Holbrook et al. (2003) determined that a technique routinely used to image the solid earth beneath the ocean, seismic reflection profiling, could also provide details of thermohaline fine structure within the ocean, at horizontal scales of tens to hundreds of kilometers and at full ocean depth. This was exciting to physical oceanogra- phers as acoustic imaging allowed the dynamics and evo- lution of thermohaline fine structure to be investigated at unprecedented resolution (Ruddick, 2003). Since then, seis- mic reflection profiling has been applied to investigate many physical oceanographic processes such as internal tides, in- ternal waves, and fronts.
Ambient noise in the ocean is another archetypal research topic that continues to receive significant attention in the TCAO. Ocean ambient noise is comprised of both natural and anthropogenic sources, and its characteristics are influ- enced by the strongly frequency-dependent nature of ab- sorption in the ocean, as well as the propagation physics of the natural ocean environment (Hildebrand, 2009). Natural sources of noise in the ocean include wind-driven surface waves, sea ice, rainfall, bubbles, biological sources (marine mammals, fish, snapping shrimp, etc.). For example, there is evidence that the disintegration of Antarctic icebergs results in seasonal increases in ocean noise levels in mid-to-equa- torial latitudes (Matsumoto et al., 2014). Ambient noise in the ocean has also been used for often clever and diverse re- mote sensing and imaging purposes, coined “ambient noise oceanography.” A recent example includes the use of seismic ambient noise to obtain high resolution tomographic maps of ocean surface waves (Sabra et al., 2005).
Much of the recent attention, however, has focused on an- thropogenic sources of sound, including sonars, seismic ex- ploration, shipping traffic, and construction, such as noise generated by wind farms and pile driving. Noise due to shipping traffic alone is thought to have increased almost 12 dB over the past few decades (Hildebrand, 2009). Some regions are particularly susceptible to increases in anthro- pogenic ambient noise, such as the Arctic. It is expected that the reduction in sea ice cover will lead to increases in oil exploration and shipping traffic, resulting in increases in anthropogenic ambient noise. An important motiva- tion for understanding the steady (Ross 2011) and perva- sive increase in anthropogenic ambient noise stems from the generally deleterious consequences to marine animals.
Understanding the background “din” these animals experi- ence is an active area of research. Describing the health of marine ecosystems in terms of their soundscape is a current hot topic, with significant overlap with research performed in the TCAB. Papers are emerging on “soundscape ecology” (Staaterman et al., 2014), with particular attention given to reef habitats as model systems, though there is still no con- vergence on a formal definition of the term with consensus amongst the various TCs.
Herman Medwin, past President of ASA, first chair of the AOTC, and founder of the ASA Medwin Prize in Acousti- cal Oceanography, has been recognized in large part for his work on bubbles in the ocean. His work includes some of the first backscattering, attenuation, and dispersion mea- surements of microbubbles in the laboratory and ocean, ex- perimental attribution to the Knudsen sea noise spectrum to bubbles produced by breaking waves, understanding bubble noise produced by rainfall, and the development of methods for measuring the vertical distribution and sizes of bubbles close to the sea surface. One could argue that Medwin didn’t leave a lot of stones unturned when it comes to understand- ing the role of bubbles in the ocean, and yet this continues to be an area of active research within the TCAO. Bubbles near the sea surface impact such critical issues such as air-sea gas exchange, affect ambient noise generated by breaking waves, modify acoustic scattering from the surface of the sea, and can have significant impact on the performance of acoustic communications systems (Deane et al., 2013).
One important recent development in “bubble acoustical oceanography” is the possibility of quantifying the flux of methane from gas seeps on the seafloor to the atmosphere and dissolved into the ocean (Weber et al., 2014), thus po- tentially affecting ocean acidification and the global carbon cycle. In fact, acoustic techniques may play a potentially critical role in mapping natural methane seeps. For example, Skarke et al. (2014) have recently used multibeam mapping to identify extensive methane gas seeps along the US Atlan- tic margin, a region not generally considered for widespread seepage. To extend this work from imaging to quantification of gas bubble size distribution it will be necessary to under- stand the role of hydrate coating and significant deviations from spherical bubbles.
Other areas of research that fall into the TCAO purview in- clude the use of active acoustic scattering and propagation techniques for the remote quantification of marine organ-
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